Sputtering of lithium

ABSTRACT

Lithium is sputtered from a target with a metallic lithium surface using an alternating sputtering potential with a frequency between about 8 and about 120 kHz, preferably about 10-100 kHz or using a DC sputtering potential and a reverse cleaning potential applied intermittently. The process can be used to apply lithium to electrochromic materials such as coatings on window glass.

This invention was made with government support under the CooperativeAgreement No. 70NANB3H1377 awarded by the National Institute ofStandards and Technology of the Department of Commerce. The UnitedStates Government has certain rights in the Invention.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 08/567,781 filed Dec. 5, 1995, now U.S. Pat. No.5,830,336.

FIELD OF THE INVENTION

The present invention relates to processes for sputtering lithium andsputtering targets useful in such processes.

BACKGROUND OF THE INVENTION

In certain industrial processes, it is necessary to add lithium to asubstrate. In particular, electrochromic devices, which are adapted tochange optical properties in response to changes in an appliedelectrical potential typically include a plurality of layersincorporating mobile lithium ions. Under the influence of an appliedpotential, the lithium ions will migrate from one layer to another. Thevarious layers are selected so that the optical properties changedepending upon the concentration of lithium in each layer. Materials ofthis nature are disclosed, for example, in U.S. Pat. No. 5,370,775.These materials can be used in optoelectronic devices such as lightmodulators, display devices and the like. Electrochromic materials canalso be used in selectively controllable window systems for variousapplications, including windows on buildings and vehicles. Certainproduction processes for making electrochromic materials requireapplication of lithium to the electrochromic material after the same isformed. As disclosed in the '755 patent, this can be accomplished byexposing the electrochromic materials to an electrolytic process usingan electrolyte bearing lithium ions. Although this process is effective,it requires exposure of the substrate bearing the electrochromic layerto a liquid electrolyte. This, in turn, can add to the cost of handlingsubstrates, particularly large substrates such as window glass panes.

It has been proposed heretofore to use sputtering to apply lithium to asubstrate such as an electrochromic substrate. In the sputtering, ionsare impelled against an exposed surface of a source or "target" formedfrom the material to be applied, as by imposing an electrical potentialbetween the target and a counterelectrode while maintaining the targetin proximity to the substrate. The energetic ions impacting on thetarget dislodge atoms of the target, commonly referred to as "adatoms",which then deposit on the substrate. Typically, such a process isconducted in a gaseous atmosphere maintained under a very lowsubatmospheric pressure. The gaseous atmosphere is ionized to form aplasma, a mixture of ionized gas atoms and free electrons. Ions of thegas form the energetic ions which bombard the target. The potentialapplied between the target and the counterelectrode ordinarily is afixed (DC) potential, wherein the target is negative with respect to thecounterelectrode, where the target is a conductive material. Analternating potential at radio frequencies (RF) typically is used whenthe target is a dielectric material. Most commonly, the radiofrequencies used for such sputtering are at the particular radiofrequencies reserved by communications authorities for industrial,scientific and medical uses, the so-called "ISM" frequencies, mosttypically about 13.56 MHZ or higher.

Targets formed from lithium compounds such as Li₂ CO₃ can besuccessfully sputtered to deposit lithium into electrochromic materials.In large scale systems, however, the RF sputtering potential requiredwith a Li₂ CO₃ target presents process problems such as nonuniformityand requires expensive equipment for generating and handling high powerRF. It would be desirable to use a sputtering target having an exposedsurface consisting essentially of pure, metallic lithium. Such ametallic lithium sputtering target at least in theory should providefaster more uniform deposition of lithium into the substrateparticularly in a relatively large-scale process. As set forth in U.S.Pat. No. 5,288,381, proposals for use of a lithium metal target surfacehave been advanced. However, there has been no practical processheretofore for sputtering lithium from a target having a metalliclithium surface. In particular, it has been impractical to sputterlithium at a reasonably fast rate from a target having metallic lithiumat its exposed surface using DC sputtering potential without damagingthe target. It has also been difficult to fabricate lithium sputteringtargets heretofore.

There have, accordingly, been substantial unmet needs for furtherimprovements in lithium sputtering processes. There have been furtherneeds for improvements in sputtering targets for use in such processesand in methods of making such targets.

SUMMARY OF THE INVENTION

The present invention addresses these needs.

One aspect of the present invention provides methods of sputteringlithium. Methods according to this aspect of the invention preferablyinclude the steps of maintaining a target having metallic lithium on anexposed surface in a substantially inert gas at subatmospheric pressuretogether with a counterelectrode and a substrate. The methods furtherinclude the step of imposing a periodically reversing electricalpotential between the target and the counterelectrode so as to form aplasma adjacent to the target and bombard the exposed surface of thetarget with ions of the gas to thereby expel lithium from the target tothe substrate. The electrical potential desirably has a reversingfrequency between about 8 kHz and about 120 kHz. Most preferably, thetarget includes a layer of metallic lithium disposed on a supportinglayer formed from a metallic material such as copper or a copper-basedalloy, the lithium being metallurgically bonded to the supporting layer.

Surprisingly, it has been found that processes employing theseconditions can allow sputtering of lithium from a lithium surface targetto proceed at a substantial rate. By contrast, attempts to sputterlithium from a target with a metallic lithium surface using anon-reversing DC potential can result in rapid destruction of the targetwhen high power levels are applied. Although the present invention isnot limited to any theory of the cause of these difficulties, it isbelieved that destruction of the target with DC potential results fromformation of a dielectric, sputter resistant layer on the targetsurface, or from impurities or defects in the target surface. It isbelieved that these layers, impurities or defects build up a staticcharge as DC sputtering continues, and that arcing occurs when thestatic charge builds to the point of dielectric breakdown of theinsulating layer. It is believed that the reversing potential causesdissipation of such charges and therefore prevents arcing. Suchdielectric layers theoretically should not form in an inert gaseousatmosphere. However, it is believed that even when substantially pureinert gases are used as the feed stock for forming the atmosphere, andeven with scrupulous attention to purging of the reaction chamber, someresidual reactive gases such as oxygen and nitrogen persist. Anyreactive gases present in the system will react with the lithium to formthe insulating films during the process. Further, it is believed thatformation of the dielectric layer can begin during exposure to airincident to handling and installation of the target and startup of thesputtering system.

Also, it is believed that a lithium layer metallurgically bonded to thesupporting layer provides a path for heat transfer from the lithiumlayer to the supporting layer having substantially lower thermalresistance than that which can be achieved by abutting contact betweenthe lithium and the supporting layer. Moreover, it is believed that thislow thermal resistance will be maintained during the process. As used inthis disclosure, the term "metallurgical bond" means an interfacebetween metallic layers at which the metallic layers are substantiallybonded to one another and in which the interface consists essentially ofmetals and intermetallic compounds. It is believed that themetallurgically-bonded interface will not be susceptible tocontamination by oxidation or other reactions with atmosphericcontaminants during the sputtering process. The methods preferablyfurther include the step of cooling the supporting layer, as by coolinga holder which is in contact with the supporting layer, so that heat iscontinually conducted from the lithium layer into the supporting layer.

Regardless of the mechanisms of operation, it has been found thatmethods according to the foregoing aspects of the present invention canbe used with surprisingly good results to sputter lithium at substantialrates.

According to a further aspect of the present invention, it has beenfound that if the target is exposed to a "clearing" potential includinga reverse-direction potential (target positive with respect tocounterelectrode) during one or more intervals in the sputteringprocess, sputtering potentials which otherwise would not be expected towork well, such as a pure forward DC sputtering potential or a lowfrequency AC sputtering potential, can be employed during the remainderof the sputtering process. The clearing potential may include one ormore periodic or aperiodic pulses of reverse-direction potentialinterspersed with forward-direction potential pulses, or may include aconventional, periodic alternating potential. Most preferably, theintervals during which the clearing potential is applied include a firstinterval before application of the sputtering potential itself. Thetarget should be maintained in the inert atmosphere during the process,from the first interval to after termination of the sputteringpotential. The process typically is conducted in an enclosed sputteringchamber and the chamber remains closed during the entire process. Anychamber opening or other exposure of the target to the ambientatmosphere desirably is followed by application of the clearingpotential. Although the present invention is not limited by any theoryof operation, it is believed that application of the reversing potentialat startup removes contaminants, such as lithium oxide leaving a verypure target surface which in turn facilitates sputtering under thesputtering potential.

Stated another way, the preferred processes according to this aspect ofthe present invention include the step of applying the clearingpotential before applying the sputtering potential, and then applyingthe sputtering potential while maintaining the target in the inertatmosphere. The exposed surface of the target is thus cleaned bysputtering during application of the cleaning potential, and thiscleaning may continue during the initial application of the sputteringpotential. The ability to start with a lithium metal target having acontaminated surface on the target offers considerable advantages inprocess design. Thus, it would require considerable care to install alithium target in a sputtering chamber without somehow contaminating itssurface, by even momentary exposure to ambient air. In accordance withthis aspect of the invention, reasonable amounts of such contaminationcan be accommodated without disrupting the sputtering process.

Application of the clearing potential during further intervals,interspersed with periods of the sputtering potential, furtherfacilitates the process. Although the present invention here again isnot limited by any theory of operation, it is believed that the clearingpotential counteracts the tendency of the lithium metal target to form adielectric layer on its exposed surface during the sputtering process.Thus, the dielectric layer is believed to form even in the presence of asubstantially inert atmosphere as used in an industrial process, due tothe inevitable presence of small amounts of contaminant gases such asoxygen and/or nitrogen, and due to the high reactivity of lithium.During the sputtering process, and particularly in a DC sputteringprocess, the dielectric layer in turn is believed to accumulate apositive charged on the side facing the plasma, which in turn can leadto arcing with the negatively-charged target if the dielectric layerbreaks down. It is believed that the clearing potential acts todissipate the positive charge. This itself suppresses arcing and alsofacilitates removal of the dielectric layer which further suppressesarcing.

In a particularly preferred process according to this aspect of theinvention, the target is exposed to an alternating potential, such asthe reversing potential discussed above, during inception of thesputtering process, and DC potentials can be employed during theremainder of the sputtering process. The target is maintained in theinert atmosphere from before termination of the alternating potential toafter termination of the DC potential. Stated another way, if thesputtering process is started using the alternating potential, it cancontinue, at reasonable speed, using a direct potential. The processtypically is conducted in an enclosed sputtering chamber and the chamberremains closed during the entire process. Any chamber opening or otherexposure of the target to the ambient atmosphere desirably is followedby application of the reversing (AC) potential. Although the presentinvention is not limited by any theory of operation, it is believed thatapplication of the reversing potential at startup removes contaminants,leaving a very pure target surface which in turn allows DC sputteringunder reasonable conditions.

Preferred processes according to these aspects of the invention providethe ability to deposit lithium uniformly over large substrates. AlthoughDC sputtering can be employed as discussed above, it is preferred toapply the a reversing potential, including periods of reverse polarity,throughout the entire sputtering process. Where the substrate includes alithium-intercalable material as discussed below, it has been found thatthe reversing potential promotes more rapid transfer of lithium into thesubstrate. The reasons for this phenomenon are not fully understood.Here again, the present invention is not limited by any theory ofoperation. However, it is believed that application of the alternatingpotential to the target and counterelectrode may also result inapplication of an alternating potential on the substrate, and that thispotential may facilitate intercalation of the lithium into thesubstrate.

The reversing potential may be a symmetrical, sinusoidal alternatingpotential, or else may have other forms such as an asymmetrical, pulsedpotential which the sputtering target is negative with respect to thecounterelectrode for the majority of the cycle and positive for theminority of the cycle. The potential more preferably has a reversingfrequency between about 10 kHz and about 100 kHz.

The counterelectrode may also include a second lithium-bearing target,in which case the second target is sputtered during one phase of thereversing potential. The substrate may include a lithium-intercalablematerial at an exposed surface, and lithium expelled from the targetdesirably intercalates into this lithium-intercalable material. Thelithium-intercalable material may be a metal chalcogenide such as anoxide of tungsten or vanadium. The lithium-intercalable material may bean electrochromic material. The process is particularly useful intreatment of relatively large substrates. Most preferably, the substrateis moved in a preselected direction of motion during the potentialapplying step so that new regions of the substrate are continuallyexposed to the expelled lithium. The substrate may be a relatively largeitem such as a sheet or pane of window glass. The substrate may havedimensions transverse to the movement direction of at least about 0.2Mand desirably about 0.2M to about 1.5M. Even larger substrates may beemployed. The target may incorporate a plurality of target elements,each such target element having an exposed surface portion. These pluraltarget elements may be retained on a single target holder. Mostdesirably, each target element includes a top layer of metallic lithiumdefining the exposed surface and a metallic supporting layer, the toplayer being metallurgically bonded to the supporting layer.

Further aspects of the present invention provide sputtering targetelements. Each such sputtering target element may include a metallicsupporting layer as discussed above together with a layer of metalliclithium overlying a front surface of the supporting layer andmetallurgically bonded to such supporting layer. The supporting layerdesirably is formed from a metal which does not tend to form alloys withlithium rapidly at elevated temperature. Desirably, the metal of thesupporting layer is selected from the group consisting of copper,copper-based alloys, nickel-plated copper and stainless steel. Indiumdesirably is present as a thin coating or interfacial layer between thelithium top layer and the metallic supporting layer, so that the lithiumlayer is bonded to the supporting layer through the indium interface.Sputtering targets according to this aspect of the present invention canbe utilized in processes as aforesaid. It is believed that the intimatemetallurgical bond between the lithium top layer and the supportinglayer materially enhances heat transfer from the lithium layer to thesupporting layer and to the other components of the apparatus. This, inturn, prevents melting of the lithium even at substantial sputteringpower levels.

Further aspects of the present invention provide methods of makingsputtering targets. Methods according to this aspect of the presentinvention desirably include the steps of providing a metallic supportinglayer, applying molten lithium to a front surface of the supportinglayer and cooling the molten lithium to thereby solidify the lithium andform a layer of lithium metallurgically bonded to the supporting layer.Most preferably, the supporting layer includes, at its front surface, ametal selected from the group consisting of copper and copper basedalloys. The step of applying molten lithium may include the step ofjuxtaposing a solid metallic lithium preferably in the form of a sheetof metallic lithium, with the supporting layer so that the solid lithiumoverlies the top surface and melting the solid lithium.

Most preferably, the molten lithium is brought to an elevatedtemperature above its melting point, desirably at least about 230° C.,and more preferably about 240 to about 280° C., and maintained at suchelevated temperature for at least about 20 minutes while in contact withthe supporting layer. Still higher temperatures, and longer holdingtimes, can also be used. Such elevated temperature and prolonged wettingtime greatly facilitates wetting of the supporting layer by the lithiumand formation of a good metallurgical bond between the lithium and thesupporting layer. Lower temperatures, typically about 190° C., can beused if the supporting layer is thoroughly cleaned before application oflithium. The step of providing a metallic supporting layer may furtherinclude the step of providing a coating of indium on the front surfaceof the supporting layer. The indium layer also promotes wetting. Thestep of melting the solid lithium can be performed by applying heat tothe supporting layer so that heat is transferred through the supportinglayer to the solid lithium. As further discussed below, these preferredarrangements provide for substantially uniform application of lithium,and substantially uniform melting of the lithium, over the extent of thefront surface. The supporting layer may have a depression in its topsurface and a ridge surrounding the depression. The step of applyingmolten lithium may be conducted so that the molten lithium completelyfills the depression and covers the ridge. This preferred methodprovides a relatively thick portion of the lithium layer in thedepression and yet provides a thin portion of the layer on the ridge.The thin portion can be retained at the outer edge of the ridge bysurface tension. This provides complete coverage of the target surfacesupport layer. The sputtering operation desirably is conducted so thatlithium is sputtered principally from the thick portion of the layer, asby aligning the thick portion of the layer with the magnetic field of amagnetron-type target holder. Thus, the target has a prolonged servicelife.

These and other objects, features and advantages of the presentinvention will be more readily apparent from the detailed description ofthe preferred embodiments set forth below, taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic perspective view depicting apparatus inaccordance with one embodiment of the invention, with portions removedfor clarity of illustration.

FIG. 2 is a diagrammatic, fragmentary sectional view taken along lines2--2 in FIG. 1.

FIG. 3 is a diagrammatic plan view taken along lines 3--3 in FIG. 2.

FIG. 4 is a graph of certain experimental results.

FIG. 5 is a diagrammatic perspective view depicting a component inaccordance with a further embodiment of the invention.

FIGS. 6 and 7 are diagrammatic perspective views depicting portions ofapparatus in accordance with further embodiments of the invention.

FIG. 8 is a graph depicting further experimental results.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Apparatus utilized in one process of the present invention includes anelectrically grounded metal-walled process chamber 10 having an upstreamend 12 and a downstream end 14. The process chamber is equipped withconventional air locks or other devices (not shown) to permit feeding ofitems to be treated into the chamber through the upstream end 12 and topermit withdrawal of the treated items at the downstream end 14. Thechamber is equipped with a substrate conveyor system schematicallyrepresented by a feed roller 16 adapted to feed flat sheet-likeworkpieces from the upstream end to the downstream end. Substrateconveyor 16, and hence the substrates treated by the approachespreferably are electrically isolated from the chamber wall 10 and henceisolated from ground potential. The chamber is also connected toconventional atmospheric control apparatus 18 adapted to fill the spacewithin chamber 10 with an inert gas at a low subatmospheric pressure.The atmospheric control apparatus may incorporate conventional elementssuch as gas supply cylinders, pressure regulators, vacuum pumps and thelike. The apparatus further includes a target element holder 20. Thetarget holder includes a generally rectangular holder plate 21 about 40cm long and about 13 cm wide. The rectangular holder plate is disposedwithin chamber 10 and extends transversely to the upstream to downstreamdirection of the chamber The target holder includes attachment devices,symbolically represented by bolts 24 extending through the holder platefor securing a base plate 22 to the holder plate. Base plate 22 isprovided with cooling fluid channels 26, which in turn are connected toa coolant supply unit 28 (FIG. 1). The coolant supply unit is adapted tocirculate a liquid through the coolant channels 26, and to maintain suchliquid at a controlled temperature, thereby controlling the temperatureof the base plate 22. Base plate 22 has a front surface 30 facing awayfrom the wall of the chamber. The target holder 20 includes conventionalmagnetron equipment 32 adapted to project magnetic flux through thefront face 30 of the base plate, and to provide such magnetic flux overa predetermined zone of the front face. This zone 34, indicated bybroken lines in FIG. 3, is generally in the shape of an oval loop or"racetrack" and is oriented with its long dimension transverse to theupstream to downstream direction of the chamber. Holder plate 21 iselectrically connected to a conductor 36, which in turn is electricallyinsulated from housing 10. Conductor 36 is connected to one side of anAC power source 38. The opposite side of the power source is connectedto ground 40 and to the metallic wall 10 of the chamber.

A sputter target element 44 in accordance with an embodiment of theinvention includes a supporting layer 46 having a front surface 48 and arear surface 50. Supporting layer 46 includes a metal at its frontsurface 48. This metal should have good thermal conductivity, but shouldnot tend to diffuse rapidly into lithium so as to contaminate lithiumremote from the supporting layer with the supporting layer metal whenthe supporting layer is held in intimate contact with lithium underelevated temperatures. The metal desirably is selected from the groupconsisting of stainless steel, copper and copper-based alloys. As usedin this disclosure, the term "copper-based alloy" means an alloyincluding more than 50% copper. Substantially pure copper is preferred.Supporting layer 46 desirably is entirely metallic. Preferably,supporting layer 46 is of a uniform composition throughout itsthickness, from its from surface 48 to its back surface 50. However,other arrangements may be used. For example, the supporting layer mayinclude metals of other compositions at locations remote from the frontsurface. Supporting layer 46 has a thin coating 54 of indium on itsfront surface 48. Coating 54 is substantially continuous over the entirefront surface 48. Each target element 44 also includes a front layer 56of metallic lithium covering the front surface of the supporting layerand hence covering the indium coating 54. As used in this disclosure,the term "metallic lithium" refers to compositions consistingessentially of metals wherein lithium is the predominant metal,accounting for more than about 75% of the metals in the composition andmost preferably accounting for about 100% of the composition.Essentially pure lithium is the most preferred form of metallic lithium,although alloys of lithium with other metals may be employed. The frontlayer 56 is metallurgically bonded to the supporting layer through theindium coating. The indium coating desirably includes only the minimumamount of indium required to form a continuous layer on the surface.Thus, the indium layer desirably is only a few microns thick. This layeris essentially invisible in the structure; it exists as a layer ofrelatively high indium concentration at the interface between themetallic lithium of the front layer and the metal of the support layer.Preferably, the lithium front layer, prior to use of the target element,is between about 1 mm and about 10 mm thick.

Each sputter target element 44 may be fabricated by first cleaning thesupporting layer 46 and etching it in an acid bath, preferablyhydrochloric acid. After removal of acid residue as by a distilled waterrinse, the supporting layer is transferred into an enclosed workingchamber such as a glove box maintained under a dry, substantially inertatmosphere such as dry, essentially oxygen-free argon. To assurecleanliness, the atmosphere in the chamber is purified by melting a massof scrap lithium within the glove box before cleaning the target. Themolten scrap lithium reacts with or "gets" any contaminant gasses fromthe chamber atmosphere. The molten scrap lithium may be maintained inthe working chamber throughout the target fabrication process. Thesupporting layer is placed on a heater, such as a laboratory hotplate,with the front surface 48 facing upwardly. The front surface should belevel, i.e., as close to a true horizontal surface as possible. Theheater is operated to supply heat to the rear surface 50 and thustransfer heat through the supporting layer. While the supporting layeris heated, a thin coating of indium is applied by depositing a smallamount of indium on the front surface. The indium tends to flow and wetthe front surface. This action may be facilitated by mechanicallyagitating the lithium with stainless steel brushes. The amount of indiumutilized need only be sufficient to fully wet the front surface, andform a substantially continuous film over the entire front surface.

After application of the indium, a layer of molten lithium is applied.The molten lithium may be applied by depositing clean, solid lithium onthe front surface. Individual pieces of lithium can be applied at spacedapart locations on the front surface. More preferably, however, solidlithium is applied as a sheet of substantially uniform thicknesscovering substantially the entire front surface of the supporting layer.The temperature of the supporting layer should be maintained as uniformas possible during the heating step. As the temperature of thesupporting layer reaches about 180° C. the solid lithium melts and formsa layer of molten lithium on the front surface. During this process, asubstantially inert wall or dam, such as a stainless steel sheet can bemaintained around the edges of the front surface to confine the moltenlithium. Alternatively, the surface tension of the molten lithium can beused to retain the molten lithium layer on the support layer. Aftermelting of the lithium and wetting of the indium-coated surface by themolten lithium, the assembly is allowed to cool under the dry, inertatmosphere. After cooling, the finished target is preserved in an inertatmosphere, as by packaging it in a sealed container under dry inertgas.

In an alternative process, the indium coating is omitted, and theheating of the supporting layer and the molten lithium is continuedafter the lithium layer has fully melted, so that the molten lithiumreaches a temperature substantially above its melting (liquidus)temperature while in contact with the supporting layer. Preferably, themolten lithium, and the supporting layer in contact therewith, areheated to an elevated temperature of at least about 230° C. and morepreferably about 240° C. to about 280° C., and maintained at thistemperature for at least about 10 minutes and more preferably at leastabout 20 minutes. Such elevated temperature treatment promotes wettingand formation of a metallurgical bond between the lithium and thesupporting layer. The indium layer can be used in with the elevatedtemperature treatment as well.

The sputtering target is secured to the base plate 22 by a layer of athermally conductive adhesive, such as a silver filled epoxy layer 58between the rear surface 50 of the target supporting layer and the frontsurface 30 of the base plate. The thermally conducting epoxy may be asilver filled epoxy. Preferably, the epoxy is capable of withstandingtemperatures up to about 180° C. and desirably can withstand even highertemperatures. Layer 58 should be as thin as possible, but should besubstantially continuous over the mating surfaces of the parts toprovide the best possible heat transfer.

As best seen in FIG. 3, a plurality of generally rectangular targetelements 44 are secured to base plate 22 in end-to-end arrangement, sothat the target elements together cover the magnetic field zone 34 ofthe target holder 20. Thus, the plural target elements form an array oftarget elements extending transverse to the upstream-to-downstreamdirection of chamber 10.

In a sputtering process according to one embodiment of the invention,target elements as discussed above are secured on target holder 20. Asubstrate 60 such as a plate or sheet of glass with a layer 62 of alithium intercalable electrochromic material is advanced through thechamber in the upstream to downstream direction by conveying device 16.As used in this disclosure, the term "electrochromic material" refers toa material or combination of materials which can be used alone or incombination with other materials to provide an electrochromic effect.Layer 62 faces towards the metallic lithium from layers of the targetelements 44. The substrate desirably moves at a rate of about 10-20cm/min, although any rate of movement can be employed depending on theamount of lithium to be deposited on the substrate. The surface of thesubstrate to be treated may be at any convenient distance from theexposed surfaces of the target elements as, for example, about 7-8 cm.Every portion of the substrate passes in front of a target element 44.Atmospheric control unit 18 is actuated to maintain an atmosphere ofsubstantially pure, dry argon at a pressure between about 1 and about100 milliTorr, and most preferably at about 10 milliTorr.

AC power unit 38 is actuated to impose an alternating potential on leads36, and hence on holder plates 21, base plates 22 and target elements44. The alternating potential has a frequency of about 120 kHz, morepreferably about 10 kHz to about 100 kHz and most preferably about 10kHz to about 40 kHz. The power source is regulated to apply asubstantially constant power level. Preferably, the power level isregulated to between about 0.2 and about 7 watts per cm² and preferablyabout 0.2 to about 3.5 watts per cm² of target element front surface.Another measure of power density in the process is power per unit lengthof the loop or racetrack region 34. Using this measure, the appliedpower should be between about 0.15 and about 4 watts per millimeter ofloop length and preferably about between 0.15 and about 2.5 watts permillimeter. The applied power converts the argon gas in the vicinity ofthe target elements to a plasma. The magnetic field provided by magneticelements 32 enhances formation of the plasma in the vicinity of thetarget elements. Thus, the gas in the chamber remote from the targetelements remains largely unionized.

During each cycle of the applied potential, the electrode assemblies,including base plates 22, go to a negative electrical potential withrespect to ground. During this phase of the cycle, positively chargedargon ions from the plasma are accelerated towards the target elementand impact upon the surface of the lithium layer, thus dislodginglithium atoms. The dislodged lithium atoms pass to the substrate andintercalate into the lithium intercalable layer 62.

If the target elements have been exposed to ambient air or otherreactive gases during installation and start up, the voltage developedacross AC power source 38 at the start of the process will be relativelyhigh. It is believed that this high voltage is caused by contaminants,such as oxides, nitrides or hydrides formed by reaction of the lithiumwith the ambient atmosphere. These contaminants can be removed bycontinued sputtering under the argon atmosphere. Even with a substantialamount of contamination, which may result from a full day's exposure ofthe target surfaces to ambient air, the sputtering operation can beconducted without appreciable arcing or destruction of the targetelements. During this initial sputtering, essentially no lithium isremoved from the target. However, upon continued operation in this mode,the contaminants are removed and the voltage drops to its normal, steadystate value, whereupon a transfer of lithium from the target elementscontinues to the normal rate for an uncontaminated target. The abilityof the process to withstand contamination of the lithium sputteringtarget surfaces is particularly important in industrial operation, as itallows reasonable handling and equipment maintenance procedures.

During the process, a substantial portion of the power applied by unit38 is dissipated as heat is applied to the lithium layers in the targetelements. The metallurgical bond at the interface between each lithiumlayer and the supporting substrate layer 46 allows good conduction ofheat from the lithium layer to a supporting layer. Heat is removed fromthe supporting layer through the silver loaded epoxy layer 58 and baseplate 22 to the cooling channels 26 and thus to the coolant circulatedby supply unit 28.

Numerous variations and combinations of the features described above canbe utilized without departing from the present invention. For example,the number of target elements, and the size of each target element, canbe varied as desired to provide sputter coating of essentially any sizesubstrate. Also, it is not essential to move the substrate during thesputtering process if all of the substrate can be accommodated in thevicinity of the sputtering target surface, or if the target itself ismoved. Inert gases other than argon can be employed. For example, heliumcan be used. Helium has an atomic mass close to that of lithium.Similarity of atomic mass promotes efficient sputtering. Substratesother than electrochromic materials can be treated. Also, essentiallyany suitable mechanical fastening arrangement can be used for securingthe base plate 22 to the electrode holder. Thus, other means such asclamps, interlocking parts or pins can be used to secure the base plateand hence the target element to the electrode assembly of the apparatus.Typically, the configuration of these elements is set by theconfiguration of the electrode holder itself.

A sputtering target element 144 in accordance with a further embodimentof the invention (FIG. 5) includes a supporting layer 146. Thesupporting layer has a top surface 148 with a depression 147 and a ridge149 surrounding the depression and defining the edges of the topsurface. A top layer 156 of metallic lithium overlies the supportinglayer. The top layer covers the entire supporting layer top surface,including depression 147 and ridge 149. The top surface of the top layeris substantially flat or bulged slightly upwardly in the center. The toplayer thus includes a relatively thick portion 155 overlying depression147 and a relatively thin portion overlying ridge 149.

A target element in accordance with this embodiment of the invention canbe made by applying molten lithium to the top surface of the supportinglayer and agitating the lithium using stainless steel brushes so as tospread the lithium over the entire top surface. Wetting of bare copper,by lithium, without an indium layer, can be promoted by such agitationand by heating the assembly well above the melting point of lithium.Thus, where no indium layer is used, the assembly desirably is heated toabout 240-280° C., most preferably about 260° C., to promote wetting.The molten lithium is effectively confined by surface tension at theouter edges of ridge 149. Because only a thin layer of lithium ispresent at the ridge, the pressure exerted by the molten lithium isminimal and is effectively counteracted by surface tension. There isnormally no need for external dams or barriers at the edges.

In use, target 144 is fastened to a base plate 122 which in turn issecured to a target holder 121. Holder 121 includes magnetic elements132 similar to those discussed above, which provide a magnetic field ina magnetic field region 134. Target 144 is secured to holder 121 so thatdepression 147 and the thick portion 155 of the top layer are alignedwith magnetic field region 134. The intensity of the plasma, and hencethe rate of sputtering are far higher rate in the magnetic field regionthan in other areas. Therefore, lithium will be sputtered principallyfrom the thick portion of the top layer. The thick portion allowsextended use of the target.

As shown in FIG. 6, two lithium-bearing targets 244 and 245 can beconnected to opposite sides of an AC power supply 238. These targets aredisposed within the chamber of sputtering apparatus as described above.During one phase of the AC power cycle, the first target 244 is negativewith respect to the second target 245, and hence lithium is sputteredfrom the first target. During this phase, the second target 245 servesas the counterelectrode. During the next phase, the second target 245 isnegative and serves as the source of sputtered lithium, whereas thefirst target serves as the counterelectrode.

As shown in FIG. 7, counterelectrodes 345 formed separately from thesputtering chamber can be used. These counterelectrodes can be formedfrom relatively inert, sputter-resistant materials such as stainlesssteel. The counterelectrodes can be disposed within the chamber adjacentto the lithium-bearing target 344. Location of the counterelectrodes canbe adjusted for optimum sputtering speed and uniformity. Thecounterelectrodes can be connected to one side of a power supply 338 andconnected though a high impedance 339, desirably about 500 ohms or more,to ground. The other side of power supply 338 is connected to thetarget, whereas the chamber wall is grounded.

The preferred embodiments discussed above utilize reversing oralternating potential (AC) throughout the entire sputtering process. Infurther embodiments of the invention, the reversing potential is appliedas a clearing potential during a first interval at the beginning of theprocess, followed by a sputtering potential in the form of a directpotential (DC) in which the target is negative and the counterelectrodeis positive. Desirably, the target remains within the protectedenvironment of the closed sputtering chamber from the beginning of thefirst interval or AC potential until the end of the DC or sputteringpotential. The DC potential may be commenced before termination of theAC potential, upon such termination or after such termination. However,any idle or no-potential time between termination of the clearing or ACpotential and commencement of the DC or sputtering potential should bebrief, desirably less than a day and more preferably less than an hour.If the chamber is opened and the target is exposed to ambient air forany appreciable time, the AC potential should be repeated. In thisarrangement, it is preferred to use AC potentials in the frequencyranges discussed above. However, if the AC potential is used only forstartup, and the potential is switched to DC before usable substratesare processed, then process uniformity during the AC portion of theoperation will be less critical. In this case, the reversing potentialcan be a radio frequency potential without impairing process uniformity.This approach is less preferred because of the other drawbacksassociated with RF apparatus.

The reversing potential employed as the clearing potential is notlimited to a conventional, fixed frequency symmetrical alternatingpotential such as a conventional sinusoidal AC. Merely by way ofexample, the clearing potential may include one or more pulses ofreverse-direction potential (target positive with respect to thecounterelectrode) interspersed with a series of forward-potential pulsesduring each said interval. The reverse potential applied during eachpulse of reverse-direction potential may be of the same magnitude as theforward potential employed during sputtering, or, preferably, of alesser magnitude. For example, where a forward DC potential of about 200volts is used for sputtering, the reverse-direction potential used inthe clearing intervals may be about 10 to about 200 volts. Also, thereverse-direction pulse may be the same length, longer, or, preferably,shorter, than the forward-potential pulses interspersed therewith. Forexample, each interval of clearing potential may includereverse-potential pulses between about 1 μs and about 10 μs longinterspersed with forward-potential pulses between about 10 μs and about100 μs long.

The sputtering potential also is not limited to a direct potential. Forexample, the sputtering potential may be an alternating potential havinga first frequency, whereas the clearing potential may be an alternatingpotential having a second, higher frequency.

As these and other variations and combinations of the features describedabove can be utilized, the foregoing description of the preferredembodiments should be taken by way of illustration rather than by way oflimitation of the invention as defined by the claims.

Certain aspects of the invention are further illustrated by thefollowing non-limiting examples:

EXAMPLE 1

A generally rectangular target element as described above, with alithium surface about 38 cm long and 12 cm wide is fabricated by castinglithium on an oxygen-free hard copper supporting layer about 3.2 mmthick. The lithium layer is about 5 mm thick. The supporting layer issecured to the backing plate of an MRC (Materials Research Corporation)903 sputtering cathode assembly using a silver-loaded epoxy. The epoxyis cured by baking at about 60° C. for three hours and the assembly isthen stored overnight at ambient temperature. The assembly is maintainedin an argon atmosphere during epoxy curing and during storage until use.

Substrates are fabricated by providing glass sheets with a thin,transparent layer of an electrically conductive oxide and thensputtering tungsten onto the oxide layer of the sheet in an oxidizingatmosphere to form a layer of WO₃. Substrates made using a tungstensputtering current of 8 amperes are referred to as "8 amp WO₃ " whereasother substrates, prepared using a tungsten sputtering current of 9amperes are referred to as "9 amp WO₃ ". The 9 amp WO₃ samples have athicker layer of WO₃ on the glass. Substrates are coated by passing themback and forth repeatedly under the lithium sputtering target whilesputtering lithium from the target. During this operation, the longdirection of the sputtering target is maintained transverse to thedirection of motion of the substrate. The substrate moves at a speed ofabout 15 cm/min. A sputtering potential is applied at 40 kHz.

The WO₃ layer on the substrate becomes darker as lithium intercalatesinto it. Accordingly, light transmission through the substrate ismeasured and the change in light transmission is used as a measure ofthe amount of lithium sputtered onto the substrate. The results areshown in FIG. 4. The process operates stably at power levels up to 550watts.

For comparison purposes, the same apparatus is used to sputter lithiumcarbonate (Li₂ CO₃) using radio frequency power. These results are alsoindicated in FIG. 4 by the curve indicated as "dTLI₂ CO₃ 700 watt . . .. "

The data shown in FIG. 4 indicate that sputtering from a metalliclithium target with 250 watts of sputtering power transfers enoughlithium to cause a 65% change in light transmission through an 8 amp WO₃layer in three passes of the substrate under the target (curve 100). Bycontrast, using 700 watts applied RF power with an Li₂ CO₃ sputteringtarget, with a similar 8 amp WO₃ layer, requires approximately 13-14passes to reach the same level of light transmission and hence the samelevel of lithiation. (curve 102)

EXAMPLE 2

Using procedures similar to those of Example 1, a series of test runsusing AC and DC potentials are made with a single target in a singlechamber. The target remains in the chamber and the chamber is maintainedunder the inert atmosphere from the beginning of the first run to theend of the last run. Here again, the lithium transfer to the glasssheets is measured by the percent light transmission (% T, FIG. 8) afterexposure; lower values of % T indicate more lithiation. The graph ofFIG. 8 shows the results for the various runs in the order in which theruns were made, with later runs to the right as seen in the drawing.Values of % T for runs with AC potential are shown as distance belowaxis 400 in FIG. 8, whereas values for runs with DC potential are shownabove the axis. In both cases, points closer to axis 400 representgreater degrees of lithiation. The first run 402 after the chamber isclosed is made using AC potential. Subsequent runs demonstrate thatalthough a reasonable degree of lithiation is achieved with the DC runs,the AC runs yield a higher degree of lithiation.

I claim:
 1. A method of sputtering lithium comprising the steps of:(a)providing a target including a top layer of metallic lithium defining anexposed surface and a metallic supporting layer, a counterelectrode anda substrate; (b) maintaining the target, counterelectrode and substratein a substantially inert atmosphere at subatmospheric pressure; and,while maintaining the target in said substantially inert atmosphere: (c)applying a sputtering potential between said counterelectrode and saidtarget, said sputtering potential including being either an alternatingpotential or a direct potential in a forward direction so that saidtarget is negative with respect to said counterelectrode, said step ofapplying a sputtering potential being performed so as to maintain aplasma adjacent to said target and sputter metallic lithium from saidtarget under the influence of said sputtering potential; and (d) duringone or more intervals prior to termination of said first potential,applying a clearing potential between said counterelectrode and saidtarget, said clearing potential being different from said sputteringpotential and including a reverse potential in a reverse directionopposite to said forward direction.
 2. A method of sputtering lithiumcomprising the steps of:(a) providing a target including a top layer ofmetallic lithium defining an exposed surface and a metallic supportinglayer, a counterelectrode and a substrate; (b) maintaining the target,counterelectrode and substrate in a substantially inert atmosphere atsubatmospheric pressure; and, while maintaining the target in saidsubstantially inert atmosphere: (c) applying a sputtering potentialbetween said counterelectrode and said target, said sputtering potentialincluding being either an alternating potential or a direct potential ina forward direction so that said target is negative with respect to saidcounterelectrode, said step of applying a sputtering potential beingperformed so as to maintain a plasma adjacent to said target and sputtermetallic lithium from said target under the influence of said sputteringpotential; and (d) cleaning said exposed surface by applying a clearingpotential between said counterelectrode and said target during a firstinterval before application of said sputtering potential, said clearingpotential being different from said sputtering potential and including areverse potential in a reverse direction opposite to said forwarddirection.
 3. A method as claimed in claim 2 further comprising the stepof applying said clearing potential during at least one additionalinterval after commencement of said step of applying said sputteringpotential.
 4. A method as claimed in claim 1 or claim 2 or claim 3wherein said step of applying said clearing potential includes the stepof applying a regular alternating potential having a substantiallyconstant frequency.
 5. A method as claimed in claim 4 wherein saidsputtering potential is a periodic alternating potential having a firstfrequency and said clearing potential is a periodic alternatingpotential having a second frequency higher than said first frequency. 6.A method as claimed in claim 4 wherein said sputtering potential is adirect potential.
 7. A method as claimed in claim 1 or claim 2 or claim3 wherein said step of applying said clearing potential includes thestep of applying a reverse potential in said reverse direction in one ormore pulses during each said interval.
 8. A method as claimed in claim 7wherein said pulses are applied as a series of reverse-potential pulsesinterspersed with a series of forward-potential pulses during each saidinterval.
 9. A method as claimed in claim 8 wherein each saidreverse-potential pulse is between about 1 μs and about 10 μs long andwherein each said forward-potential pulse is between about 10 μs andabout 100 μs long.
 10. A method as claimed in claim 8 wherein saidsputtering potential is a direct potential having a first magnitude andsaid reverse potential has a magnitude smaller than said firstmagnitude.
 11. A method as claimed in claim 2 or claim 3 wherein saidtarget has one or more lithium compounds on said exposed surface priorto said first interval and wherein said lithium compounds are at leastpartially removed from said exposed surface during said first interval.12. A method as claimed in claim 11 wherein said step of applying saidsputtering potential is commenced less than about 1 hour aftertermination of the said first interval.
 13. A method as claimed in claim1 or claim 2 or claim 3 wherein said step of maintaining said target,counterelectrode and substrate in said inert atmosphere includes thestep of maintaining said target, counterelectrode and substrate in anenclosed chamber and maintaining said chamber substantially closed frombefore termination of the first said interval until after termination ofsaid sputtering potential.
 14. A method as claimed in claim 1 or claim 2or claim 3 further comprising the step of cooling the layer of metalliclithium by cooling the metallic supporting layer so that heat isconducted from the lithium layer to the supporting layer.
 15. A methodas claimed in claim 14 wherein said layer of metallic lithium ismetallurgically bonded to said supporting layer.
 16. A method ofsputtering lithium comprising the steps of maintaining a first targetincluding a top layer of metallic lithium defining an exposed surfaceand a metallic supporting layer, said top layer being metallurgicallybonded to said supporting layer, a counterelectrode and a substrate in asubstantially inert gas at subatmospheric pressure and imposing apotential between said target and said counterelectrode so as to form aplasma adjacent to said target and bombard said exposed surface withions of said gas to thereby expel lithium from said target to saidsubstrate, said step of imposing said potential including the step ofapplying an alternating potential having a reversing frequency betweenabout 8 kHz and about 120 kHz.
 17. A method as claimed in claim 16wherein said step of imposing said potential includes the step ofapplying a direct potential, said direct potential continuing aftertermination of said alternating potential, said target being maintainedcontinuously in said inert atmosphere from the inception of saidalternating potential to termination of said direct potential.
 18. Amethod as claimed in claim 16 wherein said alternating potential isapplied throughout the entirety of said potential-imposing step.
 19. Amethod as claimed in claim 18 wherein said counterelectrode includes asecond target having metallic lithium on an exposed surface, wherebylithium will be sputtered from said second target as well as said firsttarget during application of said alternating potential.
 20. A method asclaimed in claim 16 wherein said alternating potential has a reversingfrequency between about 10 kHz and about 100 kHz.
 21. A method asclaimed in claim 16 wherein said alternating potential has asubstantially symmetrical waveform.
 22. A method as claimed in claim 16wherein said alternating potential has an asymmetrical waveform so thatsaid target is negative with respect to said counterelectrode for themajority of each cycle of said waveform.
 23. A method as claimed inclaim 16 wherein said gas consists essentially of argon or a mixture ofargon and helium.
 24. A method as claimed in claim 16 further comprisingthe step of cooling the layer of metallic lithium by cooling themetallic supporting layer so that heat is conducted from the lithiumlayer to the supporting layer.
 25. A method as claimed in claim 1 orclaim 2 or claim 16 wherein said substrate includes alithium-intercalable material at an exposed surface, and wherein saidlithium expelled from said target intercalates into saidlithium-intercalable material.
 26. A method as claimed in claim 25wherein said lithium-intercalable material is a metal chalcogenide. 27.A method as claimed in claim 26 wherein said metal chalcogenide consistsessentially of WO₃.
 28. A method as claimed in claim 25 wherein saidlithium-intercalable material is an electrochromic material.
 29. Amethod as claimed in claim 1 or claim 2 or claim 16 wherein said step ofimposing a potential is conducted so as to deliver power at a rate ofbetween about 0.2 and about 7 W per cm² of said exposed surface.
 30. Amethod of sputtering lithium comprising the steps of:(a) providing atarget including a top layer of metallic lithium defining an exposedsurface and a metallic supporting layer, a counterelectrode and asubstrate; (b) maintaining the target, counterelectrode and substrate ina substantially inert atmosphere at subatmospheric pressure; and, whilemaintaining the target in said substantially inert atmosphere: (c)applying a sputtering potential between said counterelectrode and saidtarget, said sputtering potential including being either an alternatingpotential or a direct potential in a forward direction so that saidtarget is negative with respect to said counterelectrode, said step ofapplying a sputtering potential being performed so as to maintain aplasma adjacent to said target and sputter metallic lithium from saidtarget under the influence of said sputtering potential; and (d) duringone or more intervals prior to termination of said first potential,applying a clearing potential between said counterelectrode and saidtarget, said clearing potential being different from said sputteringpotential and including a reverse potential in a reverse directionopposite to said forward direction, the method further comprising thestep of continuously moving said substrate in a direction of motionduring said step of imposing a potential to thereby expose new regionsof the substrate to said expelled lithium.
 31. A method as claimed inclaim 30 wherein said substrate and said exposed surface of said targethave dimensions transverse to said movement direction of at least about0.2 m.
 32. A method as claimed in claim 31 wherein said target includesa plurality of target elements each having an exposed surface, thesurfaces of said one or more target elements cooperatively constitutingthe exposed surface of said target.
 33. A method of sputtering lithiumcomprising the steps of:(a) providing a target including a top layer ofmetallic lithium defining an exposed surface and a metallic supportinglayer, a counterelectrode and a substrate; (b) maintaining the target,counterelectrode and substrate in a substantially inert atmosphere atsubatmospheric pressure; and, while maintaining the target in saidsubstantially inert atmosphere: (c) applying a sputtering potentialbetween said counterelectrode and said target, said sputtering potentialincluding being either an alternating potential or a direct potential ina forward direction so that said target is negative with respect to saidcounterelectrode, said step of applying a sputtering potential beingperformed so as to maintain a plasma adjacent to said target and sputtermetallic lithium from said target under the influence of said sputteringpotential; and (d) cleaning said exposed surface by applying a clearingpotential between said counterelectrode and said target during a firstinterval before application of said sputtering potential, said clearingpotential being different from said sputtering potential and including areverse potential in a reverse direction opposite to said forwarddirection, the method further comprising the step of continuously movingsaid substrate in a direction of motion during said step of imposing apotential to thereby expose new regions of the substrate to saidexpelled lithium.
 34. A method as claimed in claim 33 wherein saidsubstrate and said exposed surface of said target have dimensionstransverse to said movement direction of at least about 0.2 m.
 35. Amethod as claimed in claim 34 wherein said target includes a pluralityof target elements each having an exposed surface, the surfaces of saidone or more target elements cooperatively constituting the exposedsurface of said target.
 36. A method of sputtering lithium comprisingthe steps of maintaining a first target including a top layer ofmetallic lithium defining an exposed surface and a metallic supportinglayer, said top layer being metallurgically bonded to said supportinglayer, a counterelectrode and a substrate in a substantially inert gasat subatmospheric pressure and imposing a potential between said targetand said counterelectrode so as to form a plasma adjacent to said targetand bombard said exposed surface with ions of said gas to thereby expellithium from said target to said substrate, said step of imposing saidpotential including the step of applying an alternating potential havinga reversing frequency between about 8 kHz and about 120 kHz,the methodfurther comprising the step of continuously moving said substrate in adirection of motion during said step of imposing a potential to therebyexpose new regions of the substrate to said expelled lithium.
 37. Amethod as claimed in claim 36 wherein said substrate and said exposedsurface of said target have dimensions transverse to said movementdirection of at least about 0.2 m.
 38. A method as claimed in claim 37wherein said target includes a plurality of target elements each havingan exposed surface, the surfaces of said one or more target elementscooperatively constituting the exposed surface of said target.